1. Field of the Invention
The present invention pertains to a self-extendable shape memory alloy stent situated in a stenosal portion of tubular organs of a living body for expanding the passage of the stenosal portion and a method for fabricating the same. More specifically, the present invention is directed to a self-extendable shape memory alloy stent and a method for fabricating the same, wherein the self-extendable stent is capable of being situated to fit the shape of the passage of the stenosal portion regardless of the passage shape, whether straight (horizontal or vertical) or curved, and is prevented from any longitudinal compression or extension and any resultant length variation when the self-extendable stent is situated in the passage of the stenosal portion by use of a guide tube, while maximizing the circumferential elasticity of the self-extendable stent, thereby allowing the tasks of placing the self-extendable stent in position and expanding the stenosal portion to be performed in an efficient manner.
2. Description of the Related Art
In general, blood vessels is often blocked or constricted due to thrombus, arteriosclerosis or the like, in which event a variety of disorders may occur. In case of the blood vessels being subjected to a stenosis, it is the recent trend that the stenosis is treated through the use of a stent without resort to a surgical operation.
Conventionally, varying kinds of stents have been developed and used. Examples of the conventional stents include a self-extendable stent that has a hollow cylindrical body comprised of a super-elastic shape memory alloy wire intersectedly woven to leave a multiplicity of rhombic spaces, the opposite ends of the wire joined to each other by welding. Unless forcibly pressed by an external force, the stent normally tends to be returned back to an original shape under the action of diametrically and longitudinally exerting elastic forces.
This type of self-extendable stent is situated in a stenosal portion by heavily reducing the volume of the cylindrical body with attendant reduction of the rhombic spaces, inserting the stent up to a target stenosal portion by use of an auxiliary instrument such as a guide tube, and pushing the stent with a pusher catheter into contact with the stenosal portion, thus allowing the stent to expand the blood vessels or like tubular organs. In this regard, the stent is designed to have a diameter 10-30% greater than that of the blood vessels and a length greater than the stenosal portion of the blood vessels.
Although the self-extendable stent noted above has no difficulty in applying it to a straight or gently curved blood vessel, it is not suitable for use in a heavily winding blood vessel. In other words, the blood vessel to which the stent is applied has a tendency to be straightened by the diametrically outwardly acting elastic force of the stent without maintaining the original winding condition. This increases the length of the blood vessel and makes the passage of the blood vessel at the opposite ends of the stent become smaller than in the pre-treatment condition, contrary to the original intent.
To avoid the afore-mentioned problems, Korean Patent Application Nos. 2001-18024 and 2001-18025 filed by the present applicant disclose self-extendable stents situated in a stenosal portion of tubular organs of a living body for expanding the passage of the stenosal portion. The self-extendable stents taught in these prior patent applications are capable of maintaining the shape of a passage of the blood vessel, whether straight (horizontal or vertical) or winding, and thus minimizing unwanted deformation of the stent-situated portion of the blood vessel. This makes the self-extendable stents very useful.
More specifically, Korean Patent Application No. 2001-18024 (hereinbelow, referred to as a “first reference”) discloses a self-extendable stent of the type wherein two super-elastic shape memory alloy wires are woven in a plain weave pattern to have hooking portions and merely-intersecting portions with variable rhombic spaces left therebetween. The merely-intersecting portions are positioned between the hooking portions. The hooking portions allow the stent to be shrunk in a longitudinal direction, while the merely-intersecting portions serve to resist the longitudinal shrinkage of the stent. One of the wires is extended a diagonal length of l in a slanted circumferential direction and then turned to a different direction at a transition point. Such an extending and turning process is repeatedly conducted with respect to a multiplicity of transition points. The other of the wires is extended a diagonal length of 2 in a slanted circumferential direction and then turned to a different direction at a transition point. Such an extending and turning process is repeatedly conducted with respect to a multiplicity of transition points. In this regard, the  denotes the distance between two neighboring transition points where projecting pins are located in fabricating the stent with a jig.
On the other hand, Korean Patent Application No. 2001-18024 (hereinbelow, referred to as a “second reference”) discloses a self-extendable stent of the type wherein first and second super-elastic shape memory alloy wires are used respectively to form a primary stent member and a secondary stent member, both of which are overlaid in a radial direction. The primary stent member is fabricated by weaving the first wire in a plain weave pattern to have hooking portions and merely-intersecting portions with variable rhombic spaces left therebetween. The hooking portions allow the stent to be shrunk in a longitudinal direction, while the merely-intersecting portions serve to resist the longitudinal shrinkage of the stent. Furthermore, the secondary stent member is fabricated by diagonally extending the first wire from the top to the bottom of a cylindrical jig in a parallel relationship with the first wire of the primary stent member in such a manner that the second wire can equally divide the rhombic spaces of the primary stent member into four small spaces.
According to the self-extendable stents disclosed in the first and second references, the variable rhombic spaces formed between the hooking portions and the merely-intersecting portions are capable of being deformed by an external force, while allowing the stents to maintain elasticity in a diametrical direction. Thus, even when situated in heavily winding stenosal portion, the self-extendable stents can keep the stenosal portions of the tubular organs, such as blood vessels, a gullet, a gall duct and a urethra, in their original winding shape, while expanding the passages of the stenosal portions.
The self-extendable stents of the first and second references noted above have advantages and disadvantages in their own way.
In case of the first reference, the self-extendable stent undergoes severe contraction and extension in the longitudinal direction, because two wires are woven to have hooking portions and merely-intersecting portions. In other words, the self-extendable stent of the first reference has an advantage in that it can be flexibly deformed in conformity with the shape of a target stenosal portion. However, the self-extendable stent of the first reference is easily contracted or extended in the longitudinal direction and thus undergoes severe length variation when the stent is loaded to and unloaded from a guide tube. This reduces workability in the process of situating the stent in position. Moreover, no uniform expanding force acts on the stenosal portion if the stent is situated in the stenosal portion under a longitudinally unevenly contracted condition.
In contrast, the self-extendable stent of the second reference is free from any contraction and extension in the longitudinal direction, because the first wire constituting the primary stent member is woven to have hooking portions and merely-intersecting portions but the second wire constituting the secondary stent member has a straight portion extending from the top to the bottom with no hooking or turning. However, in case of the self-extendable stent of the second reference, the straight portion of the second wire makes it difficult for the stent to be flexed in conformity with the shape of a winding stenosal portion. Furthermore, the straight portion of the second wire reduces elasticity of the stent in a circumferential direction, which makes it difficult to effectively expand the passage of the stenosal portion and then keep the passage in the expanded condition.
Accordingly, a demand has existed for a self-extendable stent that can enjoy the advantages offered by the stents of the first and second references, namely, a self-extendable stent of the type exhibiting reduced length variation, enhanced flexibility and maximized circumferential elasticity.